Liquid crystal displays are widely applied for displaying apparatuses such as a TV set and a personal computer because the display can be operated with low electric power consumption and specifically because a thin display can be fabricated. A liquid crystal display is constituted by a liquid crystal cell having polarizing plates on both sides thereof, and the polarizing plate has a structure in which a polarizing film on which iodine or a dye is adsorbed and oriented is sandwiched between transparent resin layers on both sides. The transparent resin layers are provided to protect the polarizing film. A cellulose ester film is suitable for the resin layer and commonly used as the resin layer.
As the liquid crystal display has become popular in recent years, a thinner, larger screen or higher performance display has been desired, and according to the popularization of the liquid crystal display, a higher performance has also been desired for the cellulose ester film.
Specifically, according to the enlargement of the screen of the liquid crystal display, color unevenness or light leakage of an extent which has not been a problem has become obvious to be a problem, and a countermeasure for this problem has been required. A method to improve these performances by adjusting the properties of the cellulose ester film has been sought.
In Patent Document 1, an attention was paid on a property called as a “sound velocity” as relating to the problem of light leakage of the cellulose ester film. In this patent document, a technique to improve the quality of a display by suppressing light leakage has been proposed, in which the property of a cellulose ester film was adjusted in terms of the relationship between an amount of residual solvent and a drying temperature of the cellulose ester film. In Patent Document 2, proposed is a technique to reduce the variation of a retardation value caused by a change of circumstance such as humidity by adjusting the substitution degree of the cellulose acylate film in a prescribed value.
As a method to produce a cellulose ester film, a solution casting film formation method has been commonly earned out. In this method, a cellulose ester is dissolved in, for example, a halogen-containing solvent, the solution is cast on a belt or a drum which works as a support to form a film, and, after forming a film, the film is peeled from the support followed by drying to obtain a cellulose ester film.
Patent Documents 1 and 2 relate to a technique to control the property of a cellulose ester film by adjusting the producing condition in a solvent casting film forming method. In these patent documents, suppression of light leakage or variation of retardation has been proposed. However, when a cellulose ester film is produced in such a method, a problem of occurrence of color unevenness is newly arisen, due to uneven distribution, at the surface of the film, of a plasticizer or a UV absorber contained in the film or leaking out of an additive from the film, so called bleeding out, which may occur while the solvent is dried in the casting of drying process.
In order to reduce the color unevenness due to the uneven distribution or bleeding out of the additive, a cellulose ester film formed by a melt casing film forming method has been disclosed in Patent Document 3. In this method, a cellulose ester is heated to melt the cellulose ester and cast to form a film, without using a solvent. It has been known that, when a cellulose ester film is formed by a melt casting film forming method, color unevenness due to uneven distribution or bleeding out of an additive can be reduced because no solvent is used. However, when a cellulose ester film is formed by a melt casing film forming method, the property value of the cellulose ester film cannot be controlled by the amount of residual solvent or the film drying temperature as disclosed in Patent Document 1. Or, when a cellulose ester film is formed by a melt casing film forming method, the kind of able cellulose ester resin is limited, because it is necessary to lower the viscosity of the melt in order to maintain the mechanical strength or the flatness of the surface of the cellulose ester film. Accordingly, when a cellulose ester film is formed by a melt casing film forming method employing the method of Patent Document 3, the sound velocity of the cellulose ester film cannot be controlled, and, as the result, it was found that light leakage is caused. Further, the intensive study by the present inventor revealed that the sound velocity of a cellulose ester film affects the mechanical strength, flatness of the surface of the cellulose eater film. It was also found that the cellulose ester film produced by a melt casting film forming method is inferior with respect to color unevenness due to the uneven flatness of the film surface when compared with that of a cellulose ester film produced by a solution casting film forming method.
Accordingly, there has been no fully effective method to simultaneously overcome the problems of the light leakage and the color unevenness which have arisen according to the enlargement of the screen of the liquid crystal display.    Patent Document 1 Japanese Patent Application Publication Open to Public Inspection (hereafter referred to as JP-A) No. 2005-17574    Patent Document 2 JP-A No. 2006-30962    Patent Document 3 JP-A No. 2000-352620